In GSM, as shown in FIG. 1, a mobile switching centre MSC with associated VLR controls a plurality of Base Station Subsystems BSS 1-4 through the A interface. Each BSS controls a plurality of base stations BSNa-BSNn, each BS having an associated cell, in which may be located a number of mobile stations (not shown). The totality of cells served by the MSC is partitioned into several Location Areas LA1 LA2, etc. Each LA is served by the BSS responsible for the cells in the LA, but the cells belonging to an LA are not necessarily coincident with the cells belonging to a BSS, see, for example, LA1 which is served by BSS1-4.
In mobility management, the location of an MS is updated in the VLR when it moves from one LA to another. The VLR maintains the information for each MS in a group of LAs.
In GSM networks services, such as speech, are provided on a CS basis in which a channel or timeslot is exclusively allocated to that service. In order to set up a call from the network to a mobile station MS, a call termination procedure is carried out in which the VLR is firstly consulted as to the position of the MS, and the MS is then paged. Paging is performed by paging commands from the MSC to the BSS via the A interface, and via the radio interface over a paging channel (PCH) from the BSS to the MS. Paging through the A interface is defined, for example, in GSM 08.08., see FIG. 15 of GSM 08.08 and sections 3.1.10 and 3.2.1.19. of GSM 08.08. The CS paging request for an MS in STANDBY mode is addressed to a Location Area (LA), in which the MS was last recorded, and is distributed by the BSS in all cells belonging to this Location Area. Each LA is assigned a unique Location Area Code (LAC) by the operator and subsequently each cell within a LA is configured to have the same LAC.
The GSM system also supports data packet-switching through GPRS, for example, Internet applications. The GSM circuit-switching architecture cannot satisfy the packet-switching nature of GPRS, and GPRS requires its own transport network as shown in FIG. 2. GPRS has two control entities, firstly a support GPRS node (SGSN) and secondly, a gateway GPRS support node (GGSN) which interworks with exterior networks. The SGSN has various G interfaces with the other control entities. In particular, it has a Gs interface with the MSC/VLR, and a Gb interface with the BSS.
In GPRS networks the CS paging procedure outlined above applies in Network Operation Mode (NOM) II and III for an MS in GPRS STANDBY mode. However, in NOM I, the CS paging request has to be sent from the VLR to the SGSN through the Gs interface by a BSSAP+_PAGING_REQUEST message. Subsequently, the SGSN sends a CS paging request, comprising a PAGING_CS PDU to the BSS through the Gb interface. In this case, the CS paging request is addressed to a Routing Area (RA), which is a subset of one, and only one, LA. There may be any number, between 1 and a large number, of Routing Areas within a Location Area LA.
Only cells supporting GPRS services are configured within a RA. In a similar fashion to the LA configuration, a RA is assigned a unique Routing Area Code (RAC) by the operator and subsequently each cell within a RA is configured to have the same RAC. Cells that do not support GPRS services (non-GPRS cells) are configured only on a LA basis, while those supporting GPRS services (GPRS cells) are configured on both a LA and a RA basis. It is up to the operator to specify the number of RAs within a LA. The SGSN maintains all RA information.
In case an MS moves from a cell within a RA into a cell that belongs to another Routing Area, it will perform a Routing Area Update procedure towards the SGSN, hence the SGSN will get informed of the MS's new location, and modify the paging request accordingly. If the new RA belongs to a different LA, the MS will also perform a Location Update procedure towards the VLR, via the SGSN. Should the MS move from a GPRS cell into a non-GPRS cell that belongs to a different LA, it will perform a Location Update procedure towards the VLR, through the A-interface. However, in case the MS moves from a GPRS cell into a non-GPRS cell within the same LA, it will not initiate a Location Update procedure, hence the network (VLR/SGSN) will have no knowledge of the MS's new location. In this case, should the VLR send a CS paging request to this MS through the SGSN, the SGSN will forward the paging request to the BSS indicating paging in the last reported RA in which the MS was located. Since the MS has moved to a non-GPRS cell, it will not receive the CS paging request from the BSS. To cater for such a case, the GSM Standards specify that all non-GPRS cells within a LA are grouped by the SGSN into a ‘null RA’. Regarding the CS paging procedure at the SGSN, section 5.3 in GSM 09.18 specifies that “if within a Location Area there are cells that do not support GPRS services, the SGSN will group these cells under a ‘null RA’. The SGSN will perform the paging procedure described above within both the RA(s) derived from the location information and the ‘null RA(s)’ of the corresponding Location Area(s)”. Apart from this statement the Standards have no other references on the null RA issue. The Standards do not specify a procedure to be used by the SGSN in order to indicate to the BSS ‘paging in the null RA’. Further, it is not clear in the Standards whether the BSS should page GPRS cells only, or both GPRS and non-GPRS cells within a LA, when it receives a CS paging request from the SGSN indicating paging in the LA.